Uncovered dairy in a clean fleishig [=meat] oven — If one cooks fleishig in an oven and ensures it remains clean (i.e. the oven has no meat residue), one may bake an uncovered dry dairy product in the oven. This may be done as long as there is no uncovered meat product in the oven at the same time as the dry dairy product. "Dry" means the finished product has little or no liquid.

From what little I know of the rules of milk and meat, the reason for requiring that the dairy food be dry is that otherwise it raises steam when baking: the steam, and the food abutting it, thus re-absorbs the meat taste that got absorbed into the walls of the oven the last time liquidy meat was baked. But that would seem to be a concern any time the dairy food is liquidy while baking (once the oven is hot), whether it is liquidy when put into the oven, when removed, or any time in between. Why does the Star-K specify that the finished product must be dry?

A tip of the hat to Daniel for inspiring this question.
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msh210♦Apr 29 '13 at 19:11

On this Star-K article, they discuss the difference between cooking and baking. The idea seems to be that for foods that are baked, the liquid is absorbed into the food, whereas for things that are cooked, the liquid escapes as steam.
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DanielApr 29 '13 at 19:17

@Daniel, let me clarify, then, that I was not using their terms of art when I specified "baking" in the question above. It's just a term I use for any sort of cooking in an oven.
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msh210♦Apr 29 '13 at 19:22

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Right, I was not assuming that you were. I posted that because I think it might be the start to a possible answer. Something about the liquid being absorbed rather than released as steam.
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DanielApr 29 '13 at 19:25

1 Answer
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As explained in this Weekly Halacha column by R' Doniel Neustadt, the problem with steam is that the steam from the second dish will rise up, absorb flavor that the first dish had deposited into the oven walls, and then condense and fall back into the second dish, thus adding flavor from the first dish to the second dish. R' Neustadt records a disagreement between the authorities regarding whether the steam making deposits to and withdrawals from the oven walls is plausible in the first place:

According to She’alas Ya’avets, et al., steam dries up and evaporates in a hot oven before it reaches the walls, so the steam concern does not apply to hot ovens, and (as long as they're kept clean), they can be used serially for meat and milk dishes.

According to Igros Moshe, et al., we can't be so sure that the steam evaporates before interacting with the walls, so this concern does apply. However, Igros Moshe qualifies this concern by saying that if the heating element of the oven is on top, then we can be sure that the steam will disappear before contacting the walls of the oven.

I'm not sure how to understand this concept of steam "drying up" and "evaporating," since steam is the product of evaporation. If I'm not mistaken, for the steam (and its flavor cargo) to disappear without interacting with the oven walls, it would have to be physically vented. Perhaps that mechanism is, indeed part of the consideration.

In any case, it's clear that the authorities take into account the possibility that the heat of the oven will prevent the steam from reaching the walls in the first place. Therefore, perhaps this ruling about dishes that come out dry from the Star-K extends that concept to the condensation and return to food process using two additional mitigating factors:

For steam to condense and fall back into food, it would generally have to cool down, which is less probable in an oven that's hot enough to make it rise in the first place.

If the dish starts off dry, then we assume that it's not emitting [significant] steam to begin with, so the steam process won't affect it. If it starts off wet and ends dry, then presumably, any steam that it emits is disappearing above, since it has experienced a significant loss of liquid. So, we can assume that the steam didn't get a chance to return to the dish bearing foreign flavor from the oven walls.

I'm still waiting for a response to my email to the Star-K. I will update this answer when I receive it. In addition, this answer could be improved by looking up the primary sources, checking R' Neustadt's and my interpretations, and editing in links and quotations or direct paraphrases.